Microlithography is used for producing microstructured components, such as for example integrated circuits. The microlithographic process is carried out with a lithography apparatus, which has an illumination system and a projection system. The image of a mask (reticle) illuminated via the illumination system is in this case projected via the projection system onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, in order to transfer the mask structure to the light-sensitive coating of the substrate.
Driven by the desire for ever smaller structures in the production of integrated circuits, currently under development are EUV lithography apparatuses that use light with a wavelength in the range of 0.1 nm to 30 nm, in particular 13.5 nm. In the case of such EUV lithography apparatuses, because of the high absorption of light of this wavelength by most materials, reflective optical units, that is to say mirrors, have to be used instead of—as previously—refractive optical units, that is to say lens elements. For the same reason, beam forming and beam projection should be carried out in a vacuum.
The mirrors may for example be fastened to a supporting frame (force frame) and be configured as at least partially manipulable or tiltable in order to allow a movement of a respective mirror in up to six degrees of freedom, and consequently a highly accurate positioning of the mirrors in relation to one another, in particular in the pm range. This allows changes in the optical properties that occur for instance during the operation of the lithography apparatus, for example as a result of thermal influences, to be corrected.
For the purposes of displacing the mirrors, in particular in the six degrees of freedom, actuators which are actuated by way of a control loop are assigned to the former. An apparatus for monitoring the tilt angle of a respective mirror is provided as part of the control loop.
Document WO 03/052511 A2 discloses an imaging device in a microlithographic projection exposure apparatus. The imaging device has at least one optical element and at least one manipulator, having a linear drive, for manipulating the position of the optical element. Here, the linear drive has a driven portion and a non-driven portion which are movable relative to one another in the direction of a movement axis, wherein the portions are at least intermittently connected to one another via functional elements with an effective direction at least approximately perpendicular to the movement axis and via functional elements with an effective direction at least approximately parallel to the movement axis.
Further, it is known to capture via an optical encoder a reference pattern that is attached to a mirror. Such an optical encoder supplies voltage signals that the phase-shifted by 90° in relation to one another and are also referred to as A-signal and B-signal, but these phase-shifted voltage signals are susceptible to noise. Moreover, the optical encoder supplies an ambiguous relative position (fine position) in relation to the switch-on position or reference position, but not a unique absolute position (approximate position). Consequently, a further position sensor is used for the switch-on position.